This invention relates to electric arc furnaces, and more particularly to an improved force sensing apparatus and method for controlling travel of an electrode as it is positioned in a furnace to prevent breakage of the electrode and/or damage to components associated with the electrode.
In conventional operation of an electric arc furnace, an electric arc is established between an electrode and a conductive charge of material in the furnace to melt the furnace charge. Electrode systems in arc furnaces include an electrode, usually formed by a large rod-like member which projects downwardly into the furnace, and a movable electrode support structure, formed by a mast and an electrode holder, located outside the furnace. A drive motor lowers and raises the electrode support structure to move the electrode toward and away from the furnace charge. The electrode is connected to an electrical power supply which provides power for establishing the arc.
An arc is established as the electrode is moved toward the charge. After the arc is established, it is stabilized and maintained by controlling the position of the electrode relative to the charge in response to sensed arc conditions. Arc condition responsive drive motor control circuitry has commonly been employed for governing positioning of the electrode relative to the furnace charge in response to sensed arc conditions after an arc is established.
In the absence of an arc between the electrode and the furnace charge, properly functioning arc condition responsive drive motor controls operate the drive motor to advance the electrode toward the furnace charge. When an arc fails to be properly established as the electrode approaches the furnace charge the electrode drive motor will drive the electrode into the charge. In the absence of drive motor controls responsive to the proximity of the electrode to the furnace charge, damage to the electrode is virtually inevitable.
A common reason for failure to establish a stable arc is that the furnace charge includes materials which will not conduct a sufficient amount of current to establish an arc. In recent years the quality of scrap used for arc furnace charges has deteriorated in that increased amounts of nonconductive materials, such as pit scrap, concrete, wood, lime, coal, etc., are commonly present in furnace charges. Accordingly, the possibility of electrode damage resulting from electrodes being driven into nonconductive material in the furnace charge occurs more frequently than in the past.
Electrodes used in arc furnaces are of two basic types which are commonly referred to as "consumable" and "nonconsumable." The so-called nonconsumable electrodes are usually constructed of connected, rod-like sections formed by graphite, or an equivalent material, and are actually consumed relatively slowly during use due to arc erosion and oxidation. Usage of nonconsumable electrodes is widespread and these electrodes, because of the nature of the material from which they are constructed, are relatively easily broken by contact with nonconductive material in the furnace charge.
Nonconsumable electrodes are characterized by having relatively high compressive strength and low flexural strength. Thus these electrodes can usually withstand purely axial loads resulting from vertical contact with a horizontally disposed furnace charge during lowering of the electrode with breaking. However, when such an electrode contacts a nonhorizontal surface defined by the furnace charge, substantial transverse loads can be applied to the electrode which tend to flex it. The flexural strength of these electrodes is so low that transverse loads of relatively small magnitudes can break an electrode.
Consumable electrodes generally consist of a metallic material which is consumed during usage at a much greater rate than the nonconsumable electrodes. Consumable electrodes are structurally more durable than the nonconsumable electrodes in that they can withstand greater flexural loads, but these electrodes can also be broken or otherwise damaged when driven into engagement with the furnace charge.
Breakage of nonconsumable electrodes resulting from the electrodes being driven into engagement with the furnace charge in the absence of an arc has become a serious problem in the industry not only because of the direct costs incurred as a result of breaking the electrodes themselves, but also as a result of consequent production losses and repair and replacement costs. Electric arc furnaces are normally "three-phase" furnaces in that each furnace includes three separate electrode systems. When an electrode is broken it normally breaks off at the juncture of electrode sections nearest the electrode holder leaving a large broken-off portion in the furnace. The broken-off portion may or may not be salvagable but in any event furnace operation must be terminated to enable replacement of the broken electrode. Extended idle time of a three-phase furnace for repair and replacement of one electrode results in substantial production losses as well as exposing the unbroken electrodes of the remaining phases to excessive oxidation. Accordingly, attempts have been made to alleviated or avoid the problem of electrode breakage.
An obvious technique for preventing the electrode from being driven into the charge is to visually monitor movement of the electrode towards the charge in the event an arc is not established. Unfortunately, this technique is impractical because in order for an operator to visually monitor the position of any one electrode system in a three-phase furnace, a furnace door must be opened. In most circumstances, however, the geometry of the furnace, the charge, and the electrodes in the furnace is such that an operator cannot visually determine the distance between the electrode tip and the charge. Furthermore, opening furnace doors during operation of the furnace is a safety hazard. For these reasons visual monitoring of electrode positions is not a feasible or desirable solution to the problem.
One known prior art control system has been proposed which attempts to sense physical engagement of the electrode with the furnace charge and to retract the electrode from the charge. The proposed control system senses force variations on the electrode system which act along or parallel to the electrode axis and which may be indicative of compression of the electrode due to engagement with the charge. In this system an initial electrical signal level indicative of forces produced by electrode system weight is established and sensed by the control system. When the electrode engages the furnace charge, an axial reaction force is applied to the electrode and the signal level changes in accordance with the magnitude of the sensed change in the axial force. If the change in signal level is sufficiently great the control system causes the electrode to be lifted from the charge.
After the electrode is retracted from the furnace charge, in accordance with conventional arc furnace operation, the furnace is opened and conductive material, such as aluminum, is placed beneath the electrode, or mechanical stirring of the charge is effected to move conductive charge material beneath the electrode. The electrode is then advanced toward the charge so that an arc can be established between the conductive charge material and the electrode. This procedure usually results in the nonconductive material, which was originally engaged by the electrode, being melted or burned up after an arc has been established.
While the prior art control system represents an improvement over systems which do not employ any controls to avoid electrode breakage, the proposed control system exhibits seemingly irreconcilable sensitivity problems. On one hand the system does not appeear to be sufficiently sensitive to prevent breakage of nonconsumable electrodes in many instances where the electrode is driven into the furnace charge. At the same time the system is sensitive to sensed force changes resulting from electrode erosion and electrode system position changes and as a consequence there is a tendency for these sensed forces to falsely indicated that the electrode has engaged the furnace charge. This can result in needless withdrawal of the electrode from the charge.
It should be appreciated that the materials forming the furnace charge often provide an extremely irregular charge surface. When an electrode engages the charge the direction of the engaging force applied to the electrode by the charge can range from a direction nearly at right angles to the electrode axis to a direction along or parallel to the electrode axis, depending on the angle of engagement between the electrode and the charge. The magnitude of the axial component of a given electrode engaging force varies according to the angle at which the force is applied to the electrode. Hence engaging forces applied to an electrode which have small magnitude axial components can, in fact, have transverse components which are sufficient to break the electrode. Since the prior art control system relies on sensing only the magnitude of changes in axial forces acting on the electrode, the system inherently lack sensitivity to actual breaking forces applied to the electrode.
As indicated previously, electrodes are consumed during use which results in electrode weight reductions. The weight reductions reduce the level of the gravity forces acting on the electrode systems thus changing the net axial force applied to the electrode system. Over a period of time the force changes on an electrode system due to electrode consumption can be quite large.
Each electrode system is connected to an electrical power supply capable of producing the required arc via a power cable which is quite massive (e.g., about eight inches in diameter). Because the electrode systems must be capable of substantial vertical motion relative to the furnaces, the length of the power cables must be sufficient to accommodate the full travel of the electrode system. The cables are commonly suspended between the power supply and the electrode system and defined catenary curves which vary according to electrode system position. The axial component of the force exerted on the electrode system by the power cable can change appreciably as the electrode system changes position relative to the furnace.
The magnitude of the changes in axial forces acting on electrode systems which are attributable to electrode consumption and electrode system positon changes can be relatively great over a period of time but these force changes are nondetrimental in that they do not represent any threat of electrode breakage. The prior art system is undesirably sensitive to these nondetrimental force changes. Since the prior art control system detects magnitude changes of axial forces applied to the electrode system, and since both electrode breaking forces and nondetrimental forces have axial lines of action, the prior art control system necessarily responds to the total magnitude changes of these forces. Consequently, the sensitivity of the prior art system to actual breaking forces changes as the furnace operates and relatively frequent manual compensation is required to re-establish the desired sensitivity. As a result, while the prior art system did reduce electrode breakage somewhat, electrode breakage remained a serious problem because of the lack of sensitivity to actual electrode breaking forces, and "false tripping" of the control systems, i.e., withdrawal of the electrode from the charge in the absence of any engagement between the electrode and the charge, would become a problem. Electrode systems for arc furnaces have been subject to damage from causes other than driving the electrode into the furnace charge. When an electrode is withdrawn from a furnace, the upward travel of the electrode support structure must be limited. Limit switches governing the extent of withdrawal of electrodes from furnaces have commonly been located near the uppermost position to which the electrode support structure may be safely raised. The limit switches frequently fail because of the hostile environment in which they must be located. When the limit switches do fail the electrode support structure moves beyond the limit switches and engages mechanical stops which prevent further movement. Engagement with the stops can result in damage to the electrode support structure, the electrode system drive, and/or to drive transmission components between the drive and the support structure. The prior art control systems have not provided fail-safe electrode system operation in the event of upper limit switch failures.
When an arc has been established and stabilized, the electrode may be broken by furnace charge cave-ins during the arcing process. The electrode support structure can be damaged as a consequence of electrode breakage by a furnace charge cave-in in the absence of suitable controls. Cave-in breakage occurs when the electrode melts or burns the charge immediately underneath its nose or tip leaving unmelted charge nearby at a higher elevation than the electrode tip. The elevated unmelted charge may be unstable and thus vibrations encountered during operation of the furnace may cause some or all of it to cave-in and tumble against the electrode causing breakage. Electrodes tend to break off at a location adjacent the electrode holder so that a relatively short electrode portion remains connected to the holder and a relatively long, broken-off electrode portion remains in the furnace.
Breakage of the electrode destroys the arc and, in an effort to re-establish the arc, the arc condition responsive control circuitry causes the remaining electrode portion and its support structure to advance toward the charge to re-establish the arc. If the broken-off electrode portion is sufficiently long to stay upright due to the upper, broken end resting against the furnace roof, an arc can be struck between the broken-off electrode portion and the electrode holder, causing damage to the electrode holder and the support structure.